This invention relates to the production of extruded plastic film. More particularly, the invention relates to extruded plastic film with strips of an adhering material embedded in each surface of the film and a method and apparatus for producing such a composite plastic film.
Plastic films are widely used in commercial and home environments for enclosing various articles. The individual consumer, in particular, extensively uses plastic films for wrapping food items. The plastic film is conventionally wound on a core and packaged in film lengths ranging up to several hundred feet. Individual lengths are dispensed and cut to need.
Many consumers feed that current plastic films can be improved upon in certain respects. For instance, it is common to wrap a food article such as a sandwich in plastic film and store it it an freezer. Most plastic films sold for this purpose inherently cling to themselves. Thus, wrapping the food article with the film and then merely overlapping one portion of the plastic film with another portion results in a wrapped article which stays together because of the plastic film's inherent cling characteristics. However, when the wrapped food article is stored in the freezer, the modulus of elasticity (stiffness) of the plastic film increases which tends to cause the film to straighten out and unwrap. In fact, it is not uncommon for the plastic film to become unwrapped while in the freezer.
Using plastic film with greater inherent cling is one obvious solution. However, the cling property must not be too great due to the fact the plastic film is wound on a core during production and must be easily unrolled by the consumer during a dispensing operation. Adhesive strips positioned on the film's surface have also been suggested for holding the wrapped plastic film together. The problem encountered with such strips is they give the plastic film a slightly raised surface which becomes exacerbated when the film is wound in several layers on a core.
There is disclosed in Lefevre et al., U.S. Pat. No. 3,565,737, assigned to the same assignee as the present invention, a composite film which has increased cling. the composite film has a body portion made from low cling plastic with strips of relatively high cling or adhesive material embedded in each major surface of the body portion. The disclosed plastic film is said to have the good wrap and dispensing properties of the main body plastic and the good adhesive properties of the strip material. An illustrated apparatus for producing the composite film uses two extruders. One extruder provides a plastified film forming thermoplastic material to a sheeting die. This material forms the main body of the composite film. A second extruder feeds the adhesive material in the form of strips into the plastified mass of main body material prior to the two materials being forced out through a die lip. A controlled operation of the disclosed apparatus is difficult because of inherent design limitations. Thus, strip width variance is a problem because of injection of the adhering material into the main body mass of plastified material. Processing flexibility with regard to strip width and strip lateral placement is also very limited. Yet another problem is that the film of Lefevre, which has increased cling, tends to block (stick to itself to such a degree that it is difficult to unroll it from a roll).
Accordingly, the need remains for a more economical and efficient method and apparatus for producing composite films wherein strips of adhering materials are strategically positioned during production of the plastic film and form an improved composite plastic film which has increased cling without undue blocking tendencies.